I am now using a Rebel T1i, however I have saved almost 80% for a 24-70 f2.8L II lens. I have a f4 24-105L but sometimes shooting indoor sports is a pain ( recreational sports).
So what would you guys do? buy the f2.8 or buy the full frame and bump the ISO (5D MKII)?
what would give you better pictures?
Well, a 5dII would basically have the same focusing ability as your T1i, so while you'll gain some lower-light abilities, you won't gain anything in focus.
I know zooms are convenient, but, if you're looking for a solution for indoor sports (where the light always sucks), you might need to go the prime route. I know the 135mm F/2L is a popular one on full-frame; I'd imagine the 85mm f/1.8 and 50mm f/1.4 would probably suit your crop camera better...and they'd give you more light than just switching to a 5DII.
Depending on what else you shoot, the switch to full-frame might be worth it...but just for shooting indoor sports, it doesn't seem the right path. And I don't think the 24-70 f/2.8 will get you enough extra light to help much indoors.
the 24-105 still has IS which will still give you benefits to stoping camera shake on your end I think you will gain well over 1 stop in iso over the rebel more like 2 maybe 3
While true of subjects standing still, it will give him no gain for moving subjects, which I imagine is most of what he'd shoot in indoor sports. IS won't help there, only faster lenses. The ability to shoot hand-held at 1/10th or 1/15th of a second is only good when people are standing perfectly still